software limiting use

need help limiting the use of software by using a counter or other ways u may know

i want to limit my software from being used after limit is reached then prompt

i have a label1.caption= 0 this counter goes up by 1,2,3 etc on when it reaches 30 then then write a hidden file or to a registry and disallow user to reopen the .exe file
and prompt user you have to purchase this software you have used all your limits please help